1. Field of the Invention
The invention relates to a process for anticipating the service life of a rare gas discharge lamp and a system for anticipating the service life of a rare gas discharge lamp. The invention relates especially to a process for anticipating the service life of a rare gas discharge lamp which is a discharge lamp of the short arc type used as the light source of a projection device, a device for inspection of a liquid crystal cell or the like, a film or the like being irradiated with light and images being projected using this light. The invention furthermore relates to a system for anticipating the service life of a rare gas discharge lamp. The invention is useful, for example, in technical areas, such as in the above described projection device (projector), a device for inspection of a liquid crystal cell, stage illumination, a surgical lamp, an endoscope, a spotlight and the like as a new process for anticipating the service life of a rare gas discharge lamp which is used as a light source for them, and as a system for carrying out the process.
2. Description of Related Art
Conventionally, a rare gas discharge lamp as a discharge lamp of the short arc type is known which is used as the light source of a projection device, a device for inspection of a liquid crystal cell or the like, and in which the arc tube is filled with xenon gas. These rare gas discharge lamps have the function of producing spectra which are similar to sunlight, and are used to advantage in the above described technical areas. In these rare gas discharge lamps, the so-called flicker phenomenon occurs that, by drying out of the emitter substance of the cathode during operation, by wear of the electrodes and for similar reasons, after expiration of a certain operating interval, the arc suddenly begins to flicker and the image on the illumination surface flickers.
In these rare gas discharge lamps, as was described above, there was the disadvantage that flickering forms in the images and it becomes difficult to see the images if, for example, in the case of use as a light source of a projection device, the flicker phenomenon occurs. Furthermore, in these rare gas discharge lamps, there was the disadvantage that flickering occurs in the light which has passed through the liquid crystals and which has been projected on the test screen, and that it can no longer be correctly assessed in what area the liquid crystals are disrupted if, for example, in the case of use as a light source of a device for testing a liquid crystal cell, the flicker phenomenon occurs.
Such a flicker phenomenon in a rare gas discharge lamp can generally be determined as the magnitude of the fluctuation of the voltage of the lamp. The level (flicker level) at which the lamp becomes no longer usable due to the formation of the flicker phenomenon can be defined by the extent of the variation of the voltage even if this level differs depending on the type of lamp, the device in which the lamp is being used, the application and the like. FIG. 1 is a graph which shows the relationship between the service life of the lamp and the value (V) of the magnitude of the fluctuation of the voltage, referred to therein and below as the “swing width.” In the plot C of FIG. 1, a case of lamp operation with a rated current (80 A) is shown in which the value of the swing width of the voltage at 0.35 V is constant for up to 900 hours, and in which, after 900 hours, the value of the swing width of the voltage rises quickly. The plot C of FIG. 1 shows that the flicker level increases quickly as soon as 900 hours are exceeded even to a small degree. Plot D of FIG. 1 shows a case of operation of a lamp of the same type (with the same rated current) at the rated current (80 A) in which the value of the swing width of the voltage at 0.35 V is constant up to 800 hours, and in which after 800 hours, the value of the swing width of the voltage rises quickly. Plot D of FIG. 1, likewise, shows that the flicker level increases quickly as soon as 800 hours are exceeded even to a small degree.
In this way, the flicker phenomenon of the rare gas discharge lamp can be determined as the swing width of the lamp voltage. However, since the flicker level rises quickly in a short time when a given time is exceeded, as is shown in FIG. 1, even if the attempt is made to check the level at which use as a lamp is no longer possible at the rated current due to formation of flicker phenomenon, by examining the swing width of the voltage at the rated current, there was the disadvantage that it is extremely difficult to determine the sign of the rise of the flicker level beforehand.
As is shown in FIG. 1, even in lamps of the same type (with the same rated current) for example, due to variances of the crystal state of the electrodes, different shapes of the electrode tip as a result of the problem of processing precision, different shapes of the bulb and the like, the instant at which the swing width of the voltage changes, i.e., the instant at which the flicker level rises, is individually different, therefore not the same, depending on the lamps. There was the disadvantage that anticipating the instant of rise of the flicker level of other lamps of the same type using the instant of rise of the flicker level of a single lamp is not practical because the time errors become too large. For example, in lamps with an average time of 1000 hours up to the instant of rise of the flicker level, the width of their time errors is roughly ±200 hours.